As detailed in the Betz Handbook of Industrial Water Conditioning, 8th Edition, 1980, Betz Laboratories, Inc., Trevose, Pa., pages 85-96, the formation of scale and sludge deposits on boiler heating surfaces is a serious problem encountered in steam generation. Although current industrial steam producing systems make use of sophisticated external treatments of the boiler feedwater, e.g., coagulation, filtration, softening of water prior to its feed into the boiler system, these operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge, silts and hardness-imparting ions escape the treatment, and eventually are introduced into the steam generating system.
In addition to the problems caused by mud, sludge or silts, the industry has also had to contend with boiler scale. Although external treatment is utilized specifically in an attempt to remove calcium and magnesium from the feedwater, there is always a potential for scale formation due to residual hardness, i.e., calcium and magnesium salts. Accordingly, internal treatment, i.e., treatment of the water fed to the system, is necessary to prevent, reduce and/or retard formation of the scale imparting compounds and their resultant deposition. The carbonates of magnesium and calcium are not the only problem compounds as regards scale, but also water having high contents of phosphate, sulfate and silicate ions either occurring naturally or added for other purposes cause problems since calcium and magnesium and any iron or copper present, react with each and deposit as boiler scale. As is obvious, the deposition of scale on the structural parts of a steam generating system causes poorer circulation and lower heat transfer capacity, resulting accordingly in an overall loss of efficiency.
A so-called "all volatile treatment" (AVT) for boilers prevents boiler corrosion by maintaining the system pH in a recommended control range (about 8-10.5) that minimizes magnetite solubility, thus inhibiting boiler corrosion. AVT control provides minimal pH buffering and is relatively ineffective in preventing deposition of contaminants that either, enter with the system feedwater, or are produced by thermally driven changes in iron solubility in the feedwater. If strong ions such as sodium enter the system, they can concentrate under a previously formed deposit and result in under-deposit corrosion. Unlike AVT controlled programs, congruent phosphate controlled systems are designed to mitigate under-deposit corrosion by providing a concentrated under-deposit buffering system that prevents caustic or acidic corrosion. The higher volatility of organic pH control agents makes it difficult for amine or ammonia to help buffer under a deposit. For this reason, AVT control programs are monitored closely to assure, first, that an acidic or caustic ingress does not occur, and secondly, that any hardness or iron transport is minimized so that deposition and under-deposit corrosion sites are not encouraged.
Currently available boiler dispersants contain ionic groups which act to disperse particulates or modify crystal growth of undesirable boiler contaminants. These same ionic polymers are pH active and contain ionic additives that adversely affect AVT controlled boilers. The pH characteristic arises from two sources: first, the pH characteristic of the ionic group, and secondly, the salt used to neutralize these ionic groups. This pH activity makes it difficult to feed currently available polymers to AVT controlled boilers.